Reducing the physical size of electronic equipment in power applications is desired in order to add more features into existing products, integrate power converters in places normally unfit for such equipment, and reduce system cost. Increasing the operating frequency of the converter is a direct way of reducing the size of energy storage elements such as bulky capacitors and inductors, which usually dominate the overall converter volume. Due to reduction in energy storage requirements the transient response is dramatically increased. LED lighting applications and point-of-load (PoL) converters particularly benefit from very high frequency (VHF) converters due to size, price, and weight reduction, and faster transient response.
Conventionally, burst mode control is used to control the output voltage or current of resonant power converters. Burst mode control allows the converter designer to optimize resonant power converters for operation in one operating point. The output voltage or current is controlled by turning the resonant power converters on or off as necessary to maintain constant output voltage or current. A disadvantage of burst mode control is that the EMI performance is the same or worse compared to hard switched converters at the same modulation frequency. Typically, the modulation frequency ranges from 20 kHz to 1 Mhz.
Typically, prior art burst mode control is either hysteresis based, or pulse width modulation (PWM) based with constant switching frequency.
With hysteresis and PWM control, fast responses—ideally zero time delays—of the components of the regulation loop are required so that low cost components cannot be used.
The converter start-up circuit must also provide very little delay. As a consequence, passive start-up circuits (which have lower cost) are usually not an option.
Hysteresis based control results in tight output regulation, but requires a high cost, high performance comparator with very small propagation delays.
A conventional hysteretic burst mode control method of controlling a power converter, comprises the steps of                turning the power converter on when an absolute value of a sense voltage is less than or equal to an absolute value of a first reference voltage, and        turning the power converter off when the absolute value of the sense voltage is larger than or equal to an absolute value of a second reference voltage that is larger than the absolute value of the first reference voltage.        
Typically, the difference between the first and second reference voltages is a predetermined fixed value.